Biological adhesion impacts a wide range of phenomena with fundamental and technological importance. Adhesion in biological systems is mediated by specific adhesion receptors, and quantitative simulations suggest that the mechano-chemical properties of adhesion molecules, rather than the chemical affinity, control the dynamics of adhesion. These predictions have been corroborated with selectin adhesion molecules, which only mediate rolling. Integrins, however, can exist in leukocytes in several different active states and can mediate a wide spectrum of cellular adhesion dynamics under flow, from rolling to firm adhesion. Thus, integrins can provide a more comprehensive test of the relationship between molecular properties and the dynamics of adhesion because of their much wider dynamic spectrum. Here, we propose to engineer [unreadable]2 integrin by directed evolution and examine molecular biophysical parameters of binding to explore how changes in chemistry can be related to changes in adhesive function. In particular, we will use the ligand-binding domain (called the l-domain) of leukocyte [unreadable]2-integrin, which physiologically can convert between a passive closed configuration that mediates rolling and an open configuration that mediates firm binding. Thus, l-domain captures the entire range of possible adhesive dynamics, while also being a physiologically relevant system for the study of adhesion. Recently, our laboratory and others have shown that intermediate states of l-domain between fully closed and fully open can mediate different dynamics of rolling and that the speed of rolling is not correlated with the chemical binding affinity. Thus, the l-domain seems an ideal system to elucidate what exactly controls adhesion. With that understanding, we then propose to evolve the integrin l-domain to develop a molecular toolbox of l-domains manifesting switchable adhesion states in response to exogenous signals and enabling the manipulation of adhesion. The ultimate goal of this proposal is to understand the factors that control the dynamics of adhesion, and then develop adhesive systems with broad dynamic range and exogeneous molecular control. Switchable adhesive systems such as we will develop here will have obvious technological importance, from the development of targeted drug delivery carriers and imaging agents to methods for molecular detection. Furthermore, a fundamental understanding of biological adhesion would lead to improved therapies for diseases such as cancer and inflammation and improved methods for cell separation based on differential adhesion.